System dynamics is used to simulate the impact of feebates to encourage the sale of cleaner vehicles in southern California. With a feebate system, fees would be imposed on dirty vehicles to finance rebates for clean vehicles. The model is used to learn whether the State of California could operate a feebate system in a financially prudent manner. I consider an example in which fees are imposed on conventional vehicles, and rebates are used to encourage the sale of electric vehicles. The simulations show that feebates can be controlled despite the many uncertainties in predicting the future sales of both conventional and electric vehicles.